Engine exhaust flow from various cylinders of the engine may be imbalanced. Specifically, the imbalances in exhaust flow can result in inhomogeneous mixing of the exhaust gas within the exhaust system. This lack of mixing can be particularly disadvantageous with respect to the efficiency and performance of close-coupled catalysts, such as three way catalysts (TWCs) or selective catalyst reduction (SCR) catalysts, as there is little space for flow blending and mixing.
While different methods have been suggested to better improve cylinder to cylinder flow mixing in the exhaust system, many of them include changing the geometry of the exhaust system, including the exhaust runner design. However, engine packaging space may require exhaust runner lengths to be unequal or in an undesired position, thus limiting the ability to modify exhaust runners as desired. Further exhaust runner design can also affect the engine exhaust tone and other NVH parameters, and thus runner design may have still further constraints limiting the ability to accommodate runner design adjustments.
The inventors herein have recognized the above constraints and their interrelationship to one another, as well as various ways to address them. For example, one approach to at least partially address the above issues includes an exhaust system with an exhaust manifold having different length runners. The exhaust system also may include an emission control device housing a catalyst brick with multi-cell density. For example, one embodiment may include multi-cell density within the first catalyst brick and a uniform cell density within the second catalyst brick. Additionally, another embodiment may include multi-cell densities in both the first and second catalyst bricks.
In this way, it is possible to improve the exhaust gas flow mixing by creating specific pressure differentials within at least one catalyst brick that work in cooperation with unequal length runners of the exhaust manifold. The pressure differential can create a more homogenous exhaust flow. In one example, the pressure differential within multiple catalyst bricks can be created by further varying the cell densities among at least two catalyst bricks within a common housing in the exhaust system. As such, improved catalyst efficiency may be obtained without requiring significant physical modifications to the exhaust system, although such modifications may also be used, if desired.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.
It should be noted that still further examples are possible with different cell densities and as such combinations of the features of FIGS. 4-6 are contemplated and described herein.